Absorbent articles for feminine hygiene, such as sanitary napkins and panty liners, are widely known in the art. The purpose of such articles is to absorb and retain body fluids, which are naturally excreted from the body. Such body fluids are any water based fluids or liquids excreted from the human body such as urine, menses, serum, blood, sweat, mucous as well as other aqueous solutions generally defined as body fluids. These articles typically comprise a liquid-pervious topsheet as wearer-facing layer, a liquid-impervious backsheet as garment-facing layer and an absorbent core between topsheet and backsheet. The body fluids are acquired through the topsheet and subsequently stored in the absorbent core. The backsheet prevents the absorbed fluids from wetting the wearer's garment.
It is also widely known in the art that it is beneficial for the absorption and retention characteristics of absoroent articles when portions of the article, typically the absorbent core, comprise superabsorbent materials, such as absorbent gelling material (AGM). An example for such articles is described in WO-A-01/64154.
Recent fashion trends have led to extensive use of cloths with a certain degree of translucency, which are especially worn by younger people, in particular women. Thus, as thereby the undergarment or even certain body portions can be noticed through such translucent cloths, a need exists for providing absorbent articles, which are not noticeable through the translucent cloths, as otherwise feelings of annoyance or irritation for the wearer of such articles or other people could arise.
Generally, absorbent articles are white or lightly pink coloured, but are always constructed of a material having a certain colour. As the backsheet is the part of the article with the highest potential of being noticed, adapting its colour in a way, which harmonizes with the colour of the undergarment, would be beneficial. However, as the colour of the undergarment is usually different from garment to garment, it would be impractical if the user had to purchase absorbent articles in different colours according to the actual undergarment.
Thus, it would be beneficial to provide absorbent articles of a colour, which does not contrast with any other colour. This condition is only fulfilled by substantially transparent, colourless articles. A substantially transparent absorbent article would require to be built exclusively from transparent components. Transparent topsheets and backsheets can be provided by using transparent polymeric films or thin pigment-free nonwovens. A problem to be solved is to provide an absorbent core, which is suitable to be used in substantially transparent absorbent articles. Traditionally, absorbent cores for use in disposable absorbent articles for feminine hygiene, such as sanitary napkins or panty liners, are made of a fibrous material, such as wood pulp, cotton or rayon. Typically, those cores comprise superabsorbent material for absorbing liquids and optional materials for e.g. controlling malodours. The drawback of such cores is the fact that all those materials are usually white coloured and could thus be easily noticeable through transparent topsheets and transparent backsheets.
To address this problem, absorbent cores, which substantially consist of a polymeric material containing superabsorbent material therein, have been suggested in the art. For instance, EP-A-1,138,293 discloses a substantially transparent absorbent article, comprising an absorbent core, which consists of a hot melt adhesive containing particles of an absorbent gelling material.
Adhesive materials having liquid absorbent properties are already known in the art. U.S. Pat. No. 4,952,618 discloses hydrocolloid/adhesive compositions, containing polycationic hydrocolloid particles, for use especially in wound dressings. Another water-absorbent composition is disclosed in U.S. Pat. No. 4,977,211. This document suggests the composition to be made from three components, which are a water-absorptive resin, a polyolefin resin and an ethylene/α-olefin copolymer. WO-A-98/27559 discloses a swellable hot melt adhesive for use in the production of watertight constructions, especially watertight cables. WO-A-99/57201 discloses a thermoplastic composition, which can be a hot melt adhesive, containing particles of a superabsorbent polymer for the prevention of gel blocking in absorbent articles. EP-A-1013291 discloses a hot melt adhesive containing fluid absorbing polymers for use in e.g. absorbent products, in particular for bonding substrates such as polymeric films together. WO-A-01/64153 and WO-A-01/64155 both disclose absorbent structures made by co-extruding a thermoplastic polymer with low melting point and a superabsorbent polymer for use as absorbent component in hygiene articles. WO-A-96/16624 discloses a stretchable absorbent core for stretchable absorbent articles. It is suggested in this document to build a core from superabsorbent particles, which are incorporated into a network of a stretchable binder, which is preferably a meltblown adhesive with elastic components. U.S. Pat. No. 5,567,744 discloses a high water-absorbent composition, which is obtained by fusing or fixing a thermoplastic resin onto a surface of a high water-absorbent resin via an adhesive binder or directly, for use in e.g. sanitary absorbent articles. WO-A-96/09023 discloses an adhesive coating containing superabsorbent particles for providing thinner absorbent structures for sanitary absorbent articles.
However, conventional absorbent compositions as described above typically suffer from a contradiction between sufficient liquid-absorbency for being used in absorbent articles and appropriate viscosity for being processed on conventional converter lines for producing such absorbent articles. This contradiction is caused by the fact that the higher the amount of superabsorbent material particles used in the compositions the higher is the viscosity of such compositions. Those prior art compositions have a viscosity at 150° C. of more than 20000 centipoises (cps) when their absorbency towards aqueous fluids is high enough, i.e. they contain a high amount of superabsorbent material particles, to make them suitable for absorbent articles. This viscosity is by far too high for processing such materials on conventional converter lines. Thus, for making such materials processable by reducing the viscosity to an acceptable value of about 15000 cps, the processing temperature has to be raised to more than 150° C. This again prevents the use of certain materials, such as polyethylene, as substrate materials, e.g. backsheets, for the application of the molten absorbent resin, because these materials would melt then themselves. On the other hand, when reducing the amount of superabsorbent material to reduce the viscosity of the composition, the liquid-absorbency of the resulting compositions is too low for being used in the field of absorbent articles in the context of the present invention.
Thus, there exists a need to provide an improved liquid absorbent thermoplastic composition, which has a good water-absorbency and which are easily processable by having for example a viscosity at 150° C. of not more than 15000 cps.
A further existing need is to provide an improved liquid absorbent thermoplastic composition, which has a good water-absorbency and which are easily processable by having for example a viscosity at 150° C. of not more than 15000 cps, and which has improved mechanical properties, such as high internal cohesion of the composition.
Furthermore, there exists a need for a composition of the aforementioned kind, which is substantially transparent, so that it can be used in substantially transparent absorbent articles.
It has now surprisingly been found that all the above needs can be satisfied by selecting particular superabsorbent particles, namely superabsorbent particles having a substantially angle-lacking, preferably approximately spherical shape, and an average particle diameter in dry state of between 0.1 μm and 500 μm. Superabsorbent particles having an angle-lacking shape are known from EP-A-349,241, where a process for their production is suggested. However, there is no mentioning in the prior art towards the use of such particles in liquid-absorbent thermoplastic compositions, let alone in absorbent articles of personal hygiene.